Octagonal building



FIG. 1 is a front isometric view of the octagonal building showing my new design;

FIG. 2 is a left isometric view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a back isometric view thereof;

FIG. 5 is a right isometric view thereof;

FIG. 6 is a bottom isometric view thereof; and,

FIG. 7 is a left front isometric view thereof.

The broken lines shown in the drawings illustrate portions of the octagonal building that form no part of the claimed design. 

The ornamental design for an octagonal building, as shown and described. 